High frequency attenuator



D. L. JAFFE HIGH FREQUENCY ATTENUATOR 3 Sheets-Sheet 1 Filed Oct. 26. 1955 IN VENTOR.

HIS ATTORNEYS,

April 2, 1957 D. L. JAFFE 2,737,706

HIGH FREQUENCY ATTENUATOR Filed Oct. 26, 1353 s Shee ts-Sheet 2 #16 ATTORNEYJ- April 2, 1957 D. L. JAFFE' HIGH FREQUENCY ATTENUATOR I 3 Sheets-Sheet 3 Filed Oct. 26, 1953 5 v 6 M. w m a J A. w w 9 m 5 5 m \m 7X 1 k 9%, M .9 0 1 l ofFigs.3 and4; .2"

United States Patent HIGH FREQUENCY ATTENUATOR David Lawrence Jafre, Great Neck, N. Y., assignor to Polarad Electronics Corporation, Brooklyn, N. Y., a corporation of New York Application October 26, 1953, Serial No. 388,333

I 18 Claims. (Cl. 250-20) The present invention relates to the art concerning attenuators for electromagnetic waves and, more particularly, to attenuators for microwaves as in coaxial line or wave guide systems.

In order to adjust the magnitude of microwave energy, such as energy produced in a signal generator or other signal source, it has become common practice to utilize attenuators which either dissipate a portion of the energy while transmitting the remainder or else inhibit the flow of energy to a desired extent. One form .of such attenuator has been a section of wave guide below-cut-ofl. As is well known, if a wave guide is provided whose crosssectional dimensions are too small for the propagation of electromagnetic waves, an attenuating effect is provided along the length of such wave guide, so that if one end is coupled to an energy source and the other end is coupled to a load, the length of the below-cut-olf guide therebetween reduces the flow of energy to an extent dependent upon the length of wave guide. For most useful application of such attenuators, it is desirable that they be variable over wide ranges. In' the past, below-cut-otf wave guide attenuators have been made adjustable by providing variable lengths of below-cut-otf wave guide between the energy input and energy output points. Such attenuators have been of the rectilinear type, and the variable length of guide has been provided by either'axial adjustment of telescoping sections forming such a wave guide, or by slidably adjusting one of the coupling points along a slotted wave guide. In either case such attenuators have proven to be bulky and awkward to incorporate in wave guide coaxial line systems because .of their length and 'because of the necessity of permit-ting relatively wide adjustment of at least one coupling point.

The present invention is specifically directed towards improvements in attenuators of the below-cut-ofi wave guide type, in which the above-recited disadvantages are avoided or minimized. In particular, according to the present invention, such an attenuator is provided of the rotary type, having compact shape and dimensions, providing a minimum of spatial movement of the input and output couplings, having simple construction and adapted for wide range of adjustment.

Other features, objects and advantages of the present invent-ion will become more'fully' apparent from consideration of the following description-of preferred forms of the invention, taken in conjunction with the appended drawings, in which:

Fig. 1 is an axial cross-sectional view of one form of the present invention;

Fig. 2 is a transverse cross-sectional view of the device ofF-ig.1;- v V I Fig. 2A is a perspective view 'ofa short-circuiting'clip used in the device of Figs. land 2; r v

Fig. 3 is an axial cross-sectional view of another form of the present invention; t

Fig. 4 is a" transverse amiss-sectional view of thed'eviceofFig.3;. p

Fig. 5 is anexplod d. perspective view of the device ,Fig. 6 is a transverse cross-sectional view of another form of the present invention;

Fig. 7 is an axial cross-sectional view of the device of Fig. 6; and

Fig. 8 is an exploded perspective view of the device-of Figs. 6 and 7. V I

Referring to Figs. 1 and 2, this form of the present invention comprises a pair of easing members 11 and 12, which may be castings or machinings, or the like, each having an annular channel or groove 13 and 14 formed therein and of like dimensions, so that upon juxtaposing the casing members 11 and 1.2 coaxially the channels 13 and 14- will register with one another to form what is essentially an annular wave guide of rectangular crosssection. A coaxial line coupling 16 has its outer conductor 16a fixed in the base of channel 14 and its inner conductor 16b extends therebeyond within the channel 14. Connected between the inner and outer conductors 16a and 16b is an impedance-matching terminating resistor 17 of the microwave type, which may be supported by its rigid connecting conductors 18a and 18b, connected respectively to line conductors 16a, 16b. Resistor 17 is preferably of the type comprising a ceramic or glass rod coated with a noble metal alloy and having conductive end bands serving as terminals. Such resistors are distributed by Telewave Laboratories, Inc., of Brooklyn, N. Y. As shown in Fig. l, resistor 17 is preferably disposed with its length extending in an essentially radial direction with respect to the axisof the annular channel 14 and adjacent to and parallel to the surface 19 at which channel 14 is juxtaposed with channel 13. It will be understood that conductor 18a can, if desired, extend from the upper end of resistor 17 radially to the casing member 12 and bc connected thereto, since the outer conductor 16a is connected directly to the casing member 12. The conductors 18a, 18b and resistor 17 form a coupling loop which electromagnetically couples the wave guide formed by channels 33, 14 and the line coupling 16, to which may be joined a connecting coaxial line.

A second similar terminating resistor 21 is supported in channel 13 in a position symmetrically similar to that of and parallel to line may be connected to connect the attenuator into a system wherein it is to be used. The resistors 17, 21 preferably have resistance values matching the characteristic impedances of their respective coaxial lines.

' A control shaft 24 is provided which has a portion 26 secured to a hub 27 forming part of the casing member 12. The shaft 26 rotates freely within the casing member 11, which may be secured to a panel or housing such as 28. A speed-reducing mechanism 30 may be used where desired, or omitted for direct drive. A dial 29 is secured to the shaft 26 as by means of a nut 31 cooperating with a reduced-diameter threaded section 32 of shaft 26.

lt-will therefore be apparent that the casing member 11 section 12 is rotatable by It will be apparent that either line 16 or' line 23 may be the input to the attenuator,the other being the output. in the following, it will be assumed that line 23 isthe input. To prevent energy from the input loop 21,. 2.2a,

22b from travelling in both directions around the annular wave guide, ametallie barrier in the form of a clip 36 is secured to the base of channel 13 extending across the entire wave guide across section to form a short circuited end. As shown in Fig. 2 this clip 36 is located just beyond the resistor 17, at the proper distance to co operate with the input loop to yield satisfactory 'wave guide to coaxial line coupling. Clip 36 is shown more in detail in Fig. 2A as generally U-shaped in cross section, and is filled with microwave energy-absorbing material such as for example Metex, to provide a sliding joint and to absorb any energy which may leak between clip 36 and the wave guide wall. A second similar clip 37 may also be provided to form the other closed end of the attenuator wave guide section, if desired. Clip 37 may be omitted where clip 36 adequately serves that purpose. tom of channel 14 fixedly spaced from the position of the output loop defined by resistor 17 to yield good coupling for line 16. An adjustable length of wave guide section is thereby provided between the two movable clips 36 and 37.

In this manner input energy supplied to line 23 is coupled to the annular wave guide section formed by channels 13 and 14. An adjustable portion of that energy is picked up by the output line 16. When the resistors 17 and 21 are in the position shown in Fig. 1, there will be a maximum transfer of energy from line 23 to line 16. By rotating shaft 24 and thereby rotating casing member 12, resistor 17 is moved away from resistor 21, and an adjustable length of cut-off wave guide is interposed therebetween, creating correspondingly adjustable attenuation. It will be seen that relatively long wave guide sections may be provided in small space by virtue of the circular configuration of the wave guide, and simple means of adjustment are provided by mere rotation of a shaft, in place of formerly more complex and space-consuming racks and pinions or other necessary control arrangements for rectilinear wave guide attenuators. Attenuators of this type are useful for extended ranges of frequencies, especially between approximately 40 and 12,000 megacycles.

Figs. 3 to illustrate a modified form of the device of Figs. 1 to 2. In this case the casing 41 has an annular channel 42 having the same cross section as' the belowcut-oif wave guide. This cooperates with a fiat cover 43 to form the complete annular wave guide channel. The input coaxial line coupling 44 is mounted on cover 43 and terminates in a coupling loop 46 having an impedance-matching terminating resistor 47 as part thereof. The output coaxial line coupling 43 is secured to the casing 41 and terminates in a coupling loop 49 having an impedance-matching terminating resistor 51 as part thereof. While coupling 48 is shown as extending radially from casing 41, it may also extend axially from the base of channel 42. The casing 41 has an annular groove 58 filled with energy-absorbing material such as Metex for reducing energy leakage from the inner joint between the casing 41 and cover 43. The cover 43 has an annular groove 52 which may be filled with similar energy-absorbing material to serve as a further energyleakage-reducing means.

The cover 43 is secured to the control shaft 54 in any suitable manner as by a set screw 56. Shaft 54' may be supplied with a bearing 59 having a spring washer 61 for taking up endthrust. Mounted on shaft 54 is a collar 62 carrying a pin 63 which cooperates with fixed stops 64 secured to casing 41 to limit the rotational movement of shaft 54 and cover 43 secured thereto to the desired arcuate range of adjustment. A speedreducing mechanism 65 may be used where desired.

A pair of short-circuiting members 66 is provided for defining the ends of the below-cut-off wave guide section. As before, these short-circuiting members are preferably in the form of conductive clips blocking the wave guide channel and stuffed with energy absorbing material such as Metex to absorb any energy leaking thereby.

Alternatively, clip 37 may be secured to the bot- .In operation the form of Figs. 3 to 5 is essentially that of Fig. l. Stops 64 cooperating with the pin 63 define the minimum and maximum attenuation positions for the range of adjustment desired.

Figs. 6 to 8 illustrate another form of attenuator following the principles of the present invention. In this instance the attenuator is combined with a tunable crystal mount so that the attenuator can adjust the signal strength impressed upon the crystal which can serve as a heterodyne mixer.

This device is formed by casing member 71 having an annular channel 72 formed therein, the cross-section of this annular channel being that of the below-cut-oif wave guide serving for attenuation. The casing 71 has a central bore 73 in which is mounted a bearing 74 within which is a shaft 76. Rotatably supported on the shaft 76 is a disc or plate 96 which is supported in close contact with the casing 71 so as to form a closure for the annular channel 72. The disc 96 is formed with gear teeth 97 around its periphery which cooperate with a pinion 98 rotatably mounted on an extension 99 of the casing 71. As will be seen, rotation of disc 96 serves to tune the crystal mixer.

Also rotatably supported on the shaft 76 is a second disc or plate 103 also having gear teeth 104 at its periphery which cooperate with a pinion 105 rotatably mounted in a second extension 106 of casing 71. Disc 103 carries a coaxial line coupling 107 having an inner conductor 108 connected to a terminating resistor 109, also preferably of the above-mentioned type, which in turn is connected by a conductor 111 to the outer conductor 112 to form a signal injection probe assembly. This assembly is shown in more detail in Fig. 8.

This injection probe assembly formed by resistor 109 and its connections is inserted in the channel 72 through a slot 110 in the base of the channel 72 which extends arcuatelyabout the axis of casing 71 so as to permit adjustment of the injection probe assembly between the ends of the slot 110 by actuation of pinion 105.

Extending radially outwardly from the casing 71 is a hollow post 77 whose interior communicates with the annular channel 72 and serves to mount a second signal injection probe assembly 78 specifically a local oscillator injection probe supported therewithin and having an inner conductor 79 and terminating resistor of the type shown in Fig. 8 extending into the channel 72 to provide a coupling for injecting high frequency energy into the wave guide channel 72.

An arcuate conductor 87 of generally rectangular crosssection is supported substantially centrally of the annular channel 72 as by insulatingpins 38 extending through both the housing wall and the conductor 37. This annular conductor 87 in association with the casing 71 and cover disc 96 forms a coaxial transmission line section which is made to have adjustable length by means of an adjustable short-oircuitin'g plunger 89. Plunger 89 is formed of a pair of annular arcuate segments 91, 92 of conductive material having outer dimensions conforming to the width of the channel 72 and having an annulanly arcuate groove conforming to conductor 87 so that when the two portions 91, 92 of theplunger 89 are juxtaposed theysurround the conductor 87 as. shown in Fig. 6 and fill the channel 72. The plunger 89 thereby has sliding contact with both the. wave guide channel and the conductor 87 and provides a short circuit therebetwcen. Preferably, each of the portions 91, 92,of the plunger 39 has a thin insulating coating 94, 95 on the surface rubbing against both the casing 71 and the conductor 87 so as to provide capacitive contact rather than conductive contact therebetween; The arcuatc length of the plunger 89 is chosen in relationship to the-electrical properties of this insulating coating, which maybe .of Teflon or the like, so as to provide essentially a quarter-wave section line which acts to provide'an efiicient electrical short circuit at the face of the shorting plunger 89. The secassembly and the casing '71 so as to provide a 'tions 91, 92 of the shorting plunger 89 are secured'together and to the'rotatabl e disc 96 by insulated pins ,101, so that as the disc 96 is rotated by the knob 102 secured to the pinion 98, the position of the shorting plunger 89 is adjusted. A terminating resistor 113 is also connected between the end of the inner conductor 87 closest to probe termination for the coaxial line section 87, 71. i

The casing 71 hasa further radially-extending opening 81 surrounded by a threaded hollow post' 82 which serves as the support for a crystal 83 having a coaxial coupling 84 for-the rectified or detected crystal output. The crystal coupling is of generally conventional design ineluding capacitive by-pass for the high frequency energy and choke joints for preventing leakage of high frequency energy. The inner terminal 86 'of the crystal 83 is removably connected near the terminated end of conductor 87.

The coupling 107 provides "a means of supplying high frequency energy to the wave guide channel 72. By rotating disc 103 by the pinion 104 the separation between the coupling 107 and the crystal 83 may be adjusted so as to adjust the signal strength appearing at the crystal 83 by varying the attenuation between the coupling 107 and the crystal 83, that attenuation being a function of the length of the below-cut-off wave guide section be tween those two elements. It will be seen that the plunger 89 provides a short circuit which closes the end of the wave guide channel to prevent counter-flow of high-frequency energy to the crystal 83. The coupling 77 provides a second manner of supplying high frequency energy to the crystal 33. For example, the first coupling 107 may supply received or signal energy while the second coupling '77 may supply local oscillator energy. The crystal 83 then serves as a heterodyne mixer. The intensity of the signal from coupling 107 is adjustable by 'means of the pinion 104 to have the desired value relative to the local oscillator energy supplied to coupling 77, and the mixer crystal 83 may be set or tuned to the optimum condition by adjustment of the shorting plunger 89 by means of pinion 97 so as to give proper intermediate frequency signal.

Each of the devices described above may be enclosed in a housing having only a pair of fixed coaxial line couplings and the operating shaft projecting therefrom. These couplings may be coupled :directly to the input and output couplings of the device, as by flexible coaxial line sections. Since the movable coupling of the device is movable only through an arc of relatively small dimension, a compact device is achieved having fixed coupling points for coupling to other elements of a coaxial line or wave guide system.

Thus, each of the above described forms of the present invention incorporates a readily adjustable, simply fabricated and compact attenuator device of the belowcut-off wave guide type. I

What is claimed is:

l. A microwave device comprisng a casing having an annular channel therein, said channel having an arcuate slot through the base thereof for a portion of its circumference, a first plate rotatably mounted adjacent said casing and covering said channel to form with said casing an enclosed wave guide channel, a second plate rotatably mounted adjacent said casing on the si'de opposite said first plate and covering said slot, a coaxial transmission line coupling extending radially from said casing and having at one end a coupling loop projecting within said channel, a second coaxial transmission line coupling mounted on said second plate and having a radio frequency coupling probe assembly projecting through said slot into said channel, said probe assembly comprising an inner conductor, an outer conductor, and a terminating resistor forming an extension of said inner conductor and havingan end connected to said outer conductor, an

ar ua srm us r se m n "moun sdwi in. said channel substantially centrally of the cross sectionthereof and insulated from said casing to form an arcuate coaxial transmission line, a slidable plunger having sliding high frequency contact with both said arcuate conductor and said casing to provide a high frequency short circuit therebetween, said plunger being secured to and rotatable with said first plate, a coaxial mixer crystal mount 'secured to said casing and extending radially therefrom, a crystal having one terminal connected to said annular conductor at a point spaced from one end thereof and the other terminal extending into said mount, a terminating resistor connected between said one end of said annular conductor and said casing and respective control members coupled to said plates for rotatable adjustment thereof, whereby upon adjustment of said first plate the posit-ion of said plunger is adjusted to vary the short-circuited coaxiall transmission line between the crystaland the plunger formed by said annular conductor with the casing and the channel, and adjustment of said second plate adjusts the position of said coupling probe assembly relative to that of said coaxial coupling loop for providing adjustable attenuation therebetween.

2. A microwave device comprising a casing, having an annular channel therein, said channel having an arcuate slot through the base thereof for a portion of its circumference, a first plate rotatably mounted adjacent said casing and covering said channel to form with said casing an enclosed wave guide channel, a second plate rotatably mounted adjacent said casing on the side opposite said first plate and covering said slot, a transmission line coupling mounted on said casing with a portion in said channel, a second transmission line coupling mounted on said second plate and extending through said slot into said channel, an arcuate conductor segment mounted within said channel substantially centrally of the cross section thereof and insulated from said casing to form an arcuate coaxial transmission line, a slidable plunger having sliding high frequency contact with both said arcuate conductor and said casing to provide a high frequency short circuit therebetween, said plunger being secured to and rotatable with said first plate, a mixer crystal having one terminal connected to said arcuate conductor, and respec tive control members coupled to said plates for rotatable adjustment thereof, whereby upon adjustment of said first plate the position of said plunger is adjusted to vary the short-circuited coaxial transmission line betweenthe crystal and the plunger formed by said annular conductor with the casing and the channel, and adjustment of said second plate adjusts the position for said coupling probe assembly relative to that of said coaxial coupling loop for providing adjustable attenuation therebetween.

3. A microwave device comprising a casing having an annular channel therein, said channel havingan arcuate slot through the base thereof for a portion of its circumference, a first plate rotatably mounted adjacent said casing and covering said channel to form with said casing an enclosed wave guide channel, a second plate rotatably mounted adjacent said casing on the side opposite said first plate and covering said slot, a transmission line couplmg mounted on said casing with a portion in said channel, a second transmission line coupling mounted on said second plate and extending through said slot into said channel, a coaxial mixer crystal mount secured to said casing and extending radially therefrom, a crystal in said mount having one end electromagnetically coupled to said wave guide and respective control members coupled to said plates for rotatable adjustment thereof.

4. A microwave device comprising a casing having an annular channel therein, said channel having an arcuate slot through the base thereof for a portion-of its circumference, a first plate rotatably mounted adjacent said casing and covering said channel to form with said casing an enclosed wave guide channel, a transmission line-coupling mounted on said casing with a portion in-said chan nel, an arcuate conductor segmentomountedj 1within said answer channel substantiallycentrally of the cross section thereof and insulated from said casing to formv an arcuate coaxial transmission line, a slidable .plungerhaving sliding high frequencycontact with both said arcuate conductor and said .casing to provide a high frequency short circuit .therebetween, saidtplunger being secured to and rotatable with said first plate, a coaxial mixer crystal mount secured to saidcasing and extending radially therefrom, a crystal having one terminal connected to said annular conductorat a point spaced from one end thereof and the other terminal extending into said mount, a terminating resistor connected between said one end of said annular conductor and said casing, means for rotationally adjusting ,said plate for ,varying the position of said plunger relative to saidcrystal.

5. A microwave device comprising a casing having an annular channel therein, said channel having an arcuate slot through the .base .thereof for a portion of its circumference, a first plate .rotatably mounted adjacentsaid casing and covering said channel to form with said casing an enclosed wave guide channel, atransmission line coupling mounted on said casing with a portion in said channel, an .arcuate conductor segment mounted within said channel substantially centrally of the cross section thereof and insulated from said casing to form an arcuate coaxialtransmission line, a. slidable plunger having sliding high frequency contact with both said arcuate conductor and said casing to provide a high frequency short circuit therebetween, said plunger being secured to and rotatable with said first plate, a mixer crystal having one terminal connected to said arcuate conductor and means for rotationally adjusting said plate for varying the position of said plunger relative to said crystal.

6. A microwave device comprising a casing having an annular channel therein, said channel having an arcuate slot through the .base thereof, a first plate rotatably mounted adjacent said casing and covering said channel to form with said casing an enclosed wave guide channel dimensioned below cut-otf at the operating frequency, a second plate rotatably mounted adjacent said casing on the side opposite said first plate and covering said slot, a transmission line coupling mounted on said casing with a portion in said channel, a second transmission line coupling mounted on said second plate and having a portion extending through said slot into said channel, a coaxial mixer crystal mount secured to said casing and extending radially therefrom, "a crystal in said mount having one end electromagnetically coupled to said wave guide, a shortcircuiting member in said channel supported'by said first plate, and means for rotationally adjusting said plates to adjust said short-circuiting member and said second coupling.

7. A microwave device comprising a casing having an annular channel therein, said channel having an arcuate slotthrough the base thereof, a first plate rotatably mounted adjacent said casing and covering said channel to form with said casing an enclosed wave guide channel below cut-off at the operating frequency, a short-circuiting plunger supported by said first plate for rotation therewith and within said wave guide channel, a second plate rotatably mounted adjacent said casing on the side opposite said first plate and covering said slot, a transmission line coupling mounted on said second plate and having a coupling member projecting through said slot into said channel, a mixer crystal mounted on said casing and having one end electromagnetically coupled to said wave guide, and means for rotationally adjusting said plates for varying the positions of said short-circuiting member and of said coupling member relative to said crystal.

8. A microwave device comprising a casing having an annularchannel therein forming a wave guide below cutoff at the operating frequency, a mixer crystal mounted on said casing with one end electromagnetically coupled to said wave guide, a transmission line coupling electromagnetically coupled to said channel and movable along said channel, and means for rotationally adjusting the position of said coupling along said channel to vary the position of said coupling relative to said crystal.

microwave device as in claim 9 further comprising an arcuate conductive element mounted centrally of said channel and concentrically arcuate with respect thereto, said element being insulated-ly connected to said one casing member.

ll. A microwave device as in claim 10, further including slidable means electrically interconnecting said conductive element and said one casing member, said slidable means being fixed with respect to said other casing member whereby upon relative movement of said two casing members said slidable means forms an adjustable short for the annular transmission line formed by said conductive element and said one casing member.

l2. A microwave device as in claim ll further includ ing a crystal rectifier coupled between said conductive element and said one casing member for providing a beat frequency between waves introduced by way of said two couplings.

13. A microwave attenuator comprising a casing member having an annular channel therein of cross-sectional dimensions corresponding to those of a wave guide below cut-off at the operating frequency, a cover for such channel cooperating with said casing to enclose said channel to form an annular wave guide below cut-off, said cover being rotatably mounted with respect to said casing member about the center of curvature of said annular channel, a control member secured to said cover for rotatable adjustment thereof, a coaxial transmission line coupling secured to said casing member, and having a coupling loop at an end thereof within said wave guide channel, a second coaxial transmission line coupling mounted on said cover, and having a coupling loop at an end thereof within said wave guide channel, said two coupling loops being relatively adjustable in position by means of said control member, each of said coupling loops having a terminating impedance as a portion thereof, said terminating impedances being located within said wave guide channel, and a short-circuiting barrier extending across said wave guide channel at one position thereof to prevent passage of electromagnetic energy thereby and whereby, upon adjustment of said control member, a variable attenuation is interposed between said two transmission line couplings.

14. A microwave attenuator comprising a first casing member having an annular channel therein of cross sectional dimensions corresponding to those of a wave guide below cut-off at the operating frequency, a second casing member cooperating with said first casing member to enclose said channel and form an annular wave guide below cut-off, said two casing members being relatively rotatable about the center of curvature of said channel, a control member coupled to one of said casing members for rotatable adjustment thereof, a transmission line coupling secured to said first casing member, and having an inner conductor and an outer conductor, a microwave terminating resistor supported within said channel and having its terminals connected respectively to said inner and outer conductors, a second transmission line coupling secured to said second casing member, and having an inner conductor and an outer conductor, a second microwave terminating resistor also supported within said channel and having its terminals connected respectively to said latter inner and outer conductors, said resistors and their connections forming respective coupling loops within said channel, said two coupling loops being relatively adjustable in position by means of said control member, and a short-circuiting barrier extending across said wave guide channel at one position thereof to prevent passage of electro-magnetic energy thereby and whereby, upon adjustment of said control member, a variable attenuation is interposed between said two transmission 'line couplings.

15. A microwave attenuator comprising a first casing member having an annular channel therein of crosssectional dimensions corresponding to those of a Wave guide below cut-oif at the operating frequency, a second casing member cooperating with said first casing member to enclose said channel and form an annular wave guide below cut-off, said two casing members being relatively rotatable about the center of curvature of said channel, a control member coupled to one of said casing members for rotatable adjustment thereof, a transmission line coupling secured to said first coupling member, a second transmission line coupling secured to said second casing member, each of said couplings terminating in a coupling loop within said channel, said two coupling loops being relatively adjustable in position by means of said control member, and a short-circuiting barnier extending across said wave guide channel at one position thereof to prevent passage of electromagnetic energy thereby and whereby, upon adjustment of said control member, a variable attenuation is interposed between said two transmission line couplings.

to. A microwave attenuator comprising a first casing member having an annular channel therein of cross-sectional dimensions corresponding to those of a wave guide below cut-off at theoperating frequency, a second casing member cooperating with said first casing member to enclose said channel and form an annular wave guide below cut-off, said two casing members being relatively rotatable about the center of curvature of said channel, a control member coupled to one of said casing members for rotatable adjustment thereof, a transmission line coupling secured to said first casing member, a second transmission line coupling secured to said second casing member, each of said couplings being ele'ctromagnetically coupled to said wave guide, and having a terminating 10 energy thereby and whereby, upon adjustment of said control member, a variable attenuation is interposed between said two transmission line couplings.

17. A microwave attenuator comprising a first casing member having an annular channel therein of cross-sectional dimensions corresponding to those of a wave guide below cut-off at the operating frequency, a second casing member cooperating with said first casing member to enclose said channel and form an annular wave guide below cut-off, said two casing members being relatively rotatable about thhe center of curvature of said channel, a control member coupled to one of said casing members for rotatable adjustment thereof, a transmission line coupling secured to said first casing member, a second transmission line coupling secured to said second casing member, each of said couplings being electromagnetically coupled to said wave guide, and a short-circuiting barrier extending across said wave guide channel at one position thereof to prevent passage of electromagnetic energy thereby and whereby, upon adjustment of said control member, a variable attenuation is interposed between said two transmission line couplings.

18. A microwave attenuator comprising a first casing member having an annular channel therein of cross-sectional dimensions corresponding to those of a wave guide below cut-off at the operating frequency, a second casing member cooperating with said first casing member to enclose said channel and form an annular wave guide below cut-ofi, said two casing members being relatively rotatable about the center of curvature of said channel, a transmission line coupling secured to said first casing member, a second transmission line coupling secured to said second casing member, each of said couplings being electromagnetically coupled to said wave guide, means for rotationally adjusting the position of one of said casing members relative to the other to vary the microwave attenuation between said couplings, and short-circuiting barrier extending across said wave guide channel at one position thereof to prevent passage of electromagnetic energy thereby an-d whereby, upon adjustment of said control member, a variable attenuation is interposed between said two transmission line couplings.

References Cited in the file of this patent UNITED STATES PATENTS 2,388,049 Goode Oct. 30, 1945 2,407,267 Ginzton Sept. 10, 1946 2,525,554 Latimer Oct. 10, 1950 

